In one respect, the background of the invention concerns the use of fluorine in glass making. Fluorine in the form of a salt is a common ingredient of many glass batches. During the melting and refining of a glass batch, fluorine aids in the processing of the molten glass. For example, fluorine assists in refining of the glass melt by reducing the viscosity of the melt at relatively high temperatures and also helps release gases from the melt.
However, the use of fluorine in glass batches has several drawbacks. As a gas, fluorine is quite toxic and corrosive, and its use may become more restricted because of environmental pollution considerations. Fluorine attacks molds in which a glass melt may be cast and can severely shorten mold life.
The problems of fluorine cannot be easily overcome. Ordinarily, decreasing or deleting the amount of fluorine appreciably alters the needed or desirable properties of the glass to a point where the value and usefulness of the glass are endangered.
In another respect, the background of the invention concerns the ability of a glass to absorb X-rays. Cathode ray tubes, especially in the form of television tubes having glass faceplates, are in very common use. The faceplate has a cathodoluminescent surface. In performing its function, an electron gun of such a tube discharges X-rays toward and through the faceplate and thereby exposes a viewer of the tube to a shower of the rays. Glass faceplates of cathode ray tubes can be designed to absorb some but not all of the X-rays. The more X-rays a faceplate can absorb, the safer the tube is against X-ray exposure. The ability of a glass to absorb X-rays can be quantitatively stated by its linear absorption coefficient. This is a value expressed in reciprocable centimeters and determined for a particular wavelength of 0.6 angstrom. The linear absorption coefficient is a term established in the art and determined by known means. The higher the linear absorption coefficient is the better the absorption of X-rays by the glass.
Again, it is not a simple matter of adding more of a desired ingredient to increase X-ray absorptivity. Added amounts of any ingredient can upset the desired physical properties of a glass, such as adversely affecting its melting point, viscosity, devitrification point and rate, seal stress (stress at an area where two glasses are sealed together), softening point, annealing point, and the like.
It would, therefore, advance the art to develop a glass that is fluorine-free and has a relatively high linear absorption coefficient which, nevertheless, has acceptable workable qualities and physical properties for glass melting, refining, and processing.